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How restrictive is a 60mm diameter exhaust? (NA 13B)
Naturally Aspirated Performance ForumDiscussion of naturally-aspirated rotary performance. No Power Adders, only pure rotary power!
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How restrictive is a 60mm diameter exhaust? (NA 13B)
I'm looking to build an NA 13B.
One of my (possible/probable) limiting factors is I'm stuck with a cat-back 60mm exhaust.
I can do any custom stepping up/down necessary "header" work from the motor to the start of the 60mm exhaust pipe.
Cat is "optional", but would prefer to run a 60mm high flow cat, right before the cat back.
I want the motor to breathe well and rev freely, not just looking to squeeze as much power as I can through the 60mm pipe.
I don't want to choke the motor. Throttle response and feel is VERY important to me.
Given those exhaust variables, approximately what kind of naturally aspirated HP limit am I looking at with the 13B?
Basically, it's looking like my exhaust pipe diameter is going to be the bottle neck in my setup, so that approx HP figure would determine what type of porting and intake setup I should start considering for the build.
appreciate any input, brapbrapbrapbrap
***EDIT***
Forgot to mentions I'll be running pump gas (no E85 here), and most likely Renesis rotors so whatever octane their compression requires.
Last edited by brapbrapbrapbrap; 07-20-23 at 12:23 PM.
my David Vizard book has the answer, or an answer.
he says you want 2.2 cfm per HP. he also says pipes flow 115cfm per square inch. he then doesn't follow that up, but we can do the math and figure out the ideal pipe size for a given hp.
note that its an ideal, if you have an exhaust that doesn't flow enough, typically you would loose a little bit of power, but it will also probably be quieter. going the other way, if you have more flow than 2.2 cfm per hp, you probably won't gain any power, and it would typically be harder to make quiet.
TBD on weather the Rotary would use the 2.2cfm figure, or not, Mazda gives us this chart
anyways i set the math's up in Excel because its better at math than me
it says 60mm pipe can do 230hp, which should work well for an NA
my own car was making 274hp according to the ECU, and it was on a 60mm exhaust, post turbo backpressure was 7psi!
the new exhaust is 92mm, which is pretty ridiculous, but it should have like zero backpressure
By the way 60mm was OD no ID but I guess close enough for some desktop guess racing.
230bhp is actually just about around what I was hoping for (~200whp), so this should be just about right.
More hp the merrier but I am traction limited and don't want to ruin the car's balance.
How badly would a quality high flow 60mm cat impede my exhaust flow?
Thanks for the info regarding the noise level output, that is a major factor in my build as it's a street car and law enforcement around here aren't too friendly. Will look into that and definately consider it more when deciding on my motor build.
Other major variables for me are idle RPM, idle characteristics, and power band, but don't want to go off topic in this thread!
By the way 60mm was OD no ID but I guess close enough for some desktop guess racing.
you could use the ID, but yeah its close enough.
How badly would a quality high flow 60mm cat impede my exhaust flow?
as long as the cat can flow enough cfm, it won't be a restriction. 230hp x 2.2 = 506cfm. the problem is that, we can estimate the flow of a pipe, but we've got no way to know about a cat
as long as the cat can flow enough cfm, it won't be a restriction. 230hp x 2.2 = 506cfm. the problem is that, we can estimate the flow of a pipe, but we've got no way to know about a cat
That maakes mathematical sense, unless maybe I can maybe find flow numbers from the manufacturer for the cat, but I think that's going overboard. This it not a race car competing for a championship, just a fun street car.
I think the way I'm going to do it is tune and dyno with a test pipe and then install the cat afterwards. Unless that will drastically effect my tune or A/F ratios? I'm going with carbs though, have never dealt with that world before so I'm not even sure of what the tuning procedure is... :S
07-20-23, 11:37 AM
